The semiconductor industry has been relying on the aluminum-silicon dioxide-based system for the last 30 years. With the ever-increasing demand on performance, this system appears to be inadequate to support the future requirements. Low dielectric constant technologies, the best material of which is air itself, have been proposed to give a breath of life to the aluminum-based system. Some proposals to switch the conductor system from aluminum-based to copper-based have been made.
Most of the proposed solutions work well on computer modeling and on paper. The discussion here will concentrate on the practical aspects of the technology. The main issues addressed are: Time to market, cost of implementation, and return on investment.
The information and entertainment industries provide the driving force for the ever increasing performance enhancement requirements on integrated circuit designers and manufacturers. The industry work-horse, the aluminum-silicon dioxide system, appears to be running out of steam to support those stringent requirements. Customers are requesting higher performance and higher reliability. Based on the summary on the performance of various systems shown in Table 1, it is clear that it will be important to move away from the basic Al-SiO.sub.2 system to gain performance. Table 1 lists the relative RC time delay of various metal dielectric systems.
TABLE 1 ______________________________________ Relative Performance of Various Systems Metal-Dielectric System Relative RC Delay ______________________________________ Al--SiO.sub.2 1.0 Al--k.dbd.2 0.5 Cu--SiO.sub.2 0.67 Cu--k.dbd.2 0.31 Al--k.dbd.3.2 0.8 Al--k.dbd.3.5 0.88 ______________________________________
The best system proposed is an air-gap (air bridge) metalization system, since the relative dielectric constant of air is 1. This will provide the best RC delay that can possibly be obtained. However, this system will be difficult to manufacture and difficult to ensure quality.
In order to maintain the optimum RC delay, it is necessary to reduce capacitance. However, reducing the spacing between metal interconnects results in an increase in capacitance. Thus, a scheme is required that will enable fabrication of submicrometer metal interconnects while maintaining optimum RC delay.